1. Field of the Invention
This invention relates to differential amplifiers and more particularly to preventing input gate oxide breakdown of transistors in a differential amplifier.
2. Description of the Related Art
Differential amplifiers, also referred to as difference amplifiers and diff amps, are useful for processing low-amplitude signals in noisy environments. In general, a differential amplifier amplifies a difference in voltage between two input signals. The differential amplifier is often a building block or sub-circuit used within high-quality integrated circuit amplifiers, linear and nonlinear signal processing circuits, and even certain logic gates and digital interfacing circuits.
As noted above, differential amplifiers can be useful in digital interfacing circuits, for example, as part of an input buffer on an integrated circuit. When implemented using metal-oxide-semiconductor (MOS) technology, field effect transistors (MOSFETs) are used to form the differential amplifier. MOS transistors are either of the p-channel type (PMOS) or n-channel type (NMOS). Complementary-symmetry MOS (CMOS) technology uses both PMOS and NMOS transistors. MOS transistors have very high input impedance and consume little static power. This makes them quite useful in the design of micro-power circuits, both digital and linear. In addition, MOS transistors are also very useful in the design of amplifiers due to their extremely high input impedance.
Although quite useful, MOS transistors have a tendency to breakdown when a voltage drop across the device exceeds a process dependent level. Breakdown of a MOSFET causes permanent damage to the oxide. Typically, breakdown can occur when the voltage drop across the gate and any other node (drain, source, bulk) exceeds the nominal process voltage by 10%. For example, if the core voltage of an integrated circuit is 1.2 volts (V), a drop of 1.32 V across the gate and the drain of a transistor causes the gate oxide of the transistor to break down and the transistor to have permanent damage.
The voltage of the internal functionality of an integrated circuit, often referred to as the core voltage, often differs from external bus signals. This often happens as integrated circuits, for example, microprocessors, step down or shrink the process, lowering the required core voltage from a higher voltage, while the rest of the system components continue to run at the higher voltage.
Rather than requiring systems to conform to a single voltage level, the input pads of integrated circuits are often designed to prevent oxide breakdown. To avoid oxide breakdown, differential amplifiers in the input pads have been designed to handle large voltage differences between core and input signal voltage levels utilizing large transistors with thick gate oxide layers. In addition, a large number of transistors operating at multiple voltage levels are added to the differential amplifiers. These solutions create large areas of circuitry, increasing the size of the integrated circuit and thus increasing the cost of manufacturing. A cheaper solution is needed.
Accordingly, in one embodiment, a differential amplifier with input gate oxide breakdown avoidance amplifies a difference between two signals while maintaining voltage drops across transistors utilized in the differential amplifier to below a gate oxide breakdown level. A pull up structure allows the circuit to be utilized in IO pads of an integrated circuit and to be composed of thin oxide transistors normally only found in the core circuitry of the integrated circuit. The pull up structure is composed of three thin oxide transistors, the first transistor is connected in series with the other two, and the other two connected in parallel with respect to each other. The pull up structure prevents gate oxide breakdown of transistors utilized in the differential amplifier.
According to another embodiment, a differential amplifier has a first input terminal, a second input terminal, a first input transistor, and a second input transistor, wherein the first input terminal is connected to a gate node of the first input transistor and the second input terminal is connected to the gate node of the second input transistor. A pull-up circuit is coupled to a drain node of the first input transistor and a drain node of the second input transistor.
According to another embodiment, an integrated circuit has a means for amplifying a difference between a first and a second input signal and a means for maintaining a voltage drop across a transistor used in the means for amplifying to less than a gate oxide breakdown level.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. As will also be apparent to one of skill in the art, the operations disclosed herein may be implemented in a number of ways, and such changes and modifications may be made without departing from this invention and its broader aspects. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.